1. Field of the Invention
The present invention is related generally to thermometers and, more particularly, to a thermometer of the radiation type which measures temperatures by detecting infrared radiation emitted by a subject, such as the eardrum.
2. Description of the Prior Art
Japanese Patent Application Kokai Nos. 61-117422 ('422) and 2-28524 ('524) have proposed thermometers of this type for use in measuring the temperature of patients in hospitals because the time for taking the temperature is reduced very much. The thermometers have a metallic tube attached to the front of an infrared sensor, such as a thermopile, and an adiabatic material for surrounding the entire metal tube to form a probe. This probe is inserted into the external auditory canal to sense the infrared radiation from the eardrum for taking the temperature. In order to assure the accuracy of clinical thermometers, the '422 has proposed to preliminarily heat the measuring system to a constant temperature (36.5 degrees C.) to eliminate various errors. The '524 has proposed to assure the accuracy by means of various correction devices.
However, the above thermometers have the following disadvantages:
The '422 device requires:
(1) a time for preliminary heating;
(2) a high-precision heat control unit which makes the circuit complicated and the unit manufacturing cost high; and
(3) a high power supply for driving the heat control unit, resulting in the bulky device.
The '524 device does not employ any heat control unit and is free of the above disadvantages, but, where there is thermal unbalance in the system, it employs a temperature sensor attached to the front tube of the infrared sensor to compute the temperature using the measurement for correction. However, it still has the following disadvantages:
(1) The thermal unbalance indicates the presence of thermal gradient in the metal tube. Consequently, taking a measurement at a point as the representative temperature of the metal tube brings about a new error.
(2) To avoid such an error, a separate sensor is provided near the infrared sensor to compute a difference in temperature between them while a critical temperature is set to issue a measurement permit. However, their circuitry is so complicated as to push up the unit manufacturing cost and limit the measuring range.